1. Field of the Invention
The invention relates to a filter designed for signals in a band of high frequencies such as microwaves or millimeter waves, and a method of arranging resonators which constitute the filter.
2. Description of the Related Art
In the field of communications, filters intended for signals in a band of high frequencies such as microwaves or millimeter waves have been heretofore developed. As the types of such filters, there are known, for example, a waveguide filter, a waveguide-type dielectric filter, and the like.
FIG. 11 shows a configuration of a conventional waveguide filter. The waveguide filter includes a wiring board 110, and a plurality of resonators 101 to 105 each comprising a waveguide, which are arranged in series on the wiring board 110. A signal input 111 and a signal output 112 are provided on one and the other ends, respectively, of the wiring board 110. The resonators 101 to 105 are arranged between the signal input 111 and the signal output 112.
FIG. 12 shows coupling of the resonators 101 to 105 of the waveguide filter. In the waveguide filter, the resonators 101 to 105 are electromagnetically coupled in series, and the adjacent resonators 101 and 102, 102 and 103, 103 and 104, and 104 and 105 have coupling coefficients of k12, k23, k34, and k45, respectively. The waveguide filter allows the passage of signals in a band of resonance frequencies of the resonators 101 to 105 electromagnetically coupled, and reflects signals outside this band.
The prior arts of the filter including a plurality of resonators connected in series as mentioned above include filters disclosed in Japanese Unexamined Patent Application Publication No. 2002-43807 and Japanese Unexamined Patent Application Publication No. 2002-26611, for example. Japanese Unexamined Patent Application Publication No. 2002-43807 discloses an example of a waveguide-type dielectric filter, which includes a dielectric block in the shape of a rectangular parallelepiped including a plurality of resonant elements, and a wiring board having the dielectric block mounted thereon. Japanese Unexamined Patent Application Publication No. 2002-26611 discloses an example of a dielectric filter having a configuration in which through holes are used as a sidewall of a waveguide.
Recently, frequencies of signals for use in communications equipment have become increasingly higher, and a filter having excellent frequency characteristics has been also desired. Thus, for example to implement a band-pass filter which allows the passage of a specific frequency band alone, an attenuation pole (i.e., a trap) can be formed in a range other than a pass band so as to improve attenuation characteristics. For instance when two signal propagation paths 121 and 122 are connected in parallel between the signal input 111 and the signal output 112 as shown in FIG. 13, a phase difference of π arising between the two propagation paths 121 and 122 allows electromagnetic waves to cancel each other out, thus forming the attenuation pole. However, the conventional waveguide filter has a structure including the waveguides connected in series as shown in FIG. 11, not a structure adapted to form a plurality of propagation paths, so that the filter cannot produce the attenuation pole.